1. Field of the Invention
The invention relates to a hydraulic power steering system for motor vehicles having a source for a hydraulic medium, in particular an oil pump, which can be driven during operation by a drive device, in particular by an internal combustion engine of the vehicle or by an electric motor, having a steering transmission or steering gear assembly, and having a line system for the hydraulic medium connecting the source to the steering transmission.
2. Description of the Prior Art
With known power steering systems of this kind, the line system connecting the oil pump to the steering transmission simply comprises a rubber hose. The length of this hose depends on the respective vehicle circumstances and in particular on the specific arrangement of the individual components e.g. in the engine space of the vehicle. Typical hose lengths are, for example, in the region of 50 cm, with shorter or longer connection hoses, however, also being able to be used in dependence on the respective circumstances.
It is problematic with such systems that speed fluctuations, e.g. of the internal combustion engine or of the electric motor, have an effect on the oil pump integrated in the drivetrain of the vehicle such that the conveying performance of the oil pump varies accordingly in time, which also has the consequence of fluctuations called rotary oscillations in the oil flow from the oil pump to the steering transmission or in the oil pressure prevailing in this hydraulic system. These fluctuations are felt by the driver as vibrations on the steering wheel and are perceived as irritating. These flow variations or pressure variations in the hydraulic medium which are ultimately induced by the drive device are also called “ripples” or “speed ripples.” The drive device is also simply called a “motor” in the following. This in particular includes—but is not exclusive—an internal combustion engine provided for the propulsion drive of the vehicle or the electric motor of an electric hydraulic power steering (EHPS) system in which the motor drives the oil pump.
The problems of these vibrations perceptible at the steering wheel are currently increasing in relevance since an attempt is being made in particular in conjunction with diesel engines to keep the idling speed as low as possible. The fluctuations in the oil flow or oil pressure resulting in the irritating steering wheel vibrations are the more pronounced, the lower the speed of the motor driving the oil pump is.
An attempt can generally be made to combat these irritating steering wheel vibrations using improved interfaces, such as of the motor bearings, of the belt pulley, etc., of the motor and/or using an improved motor management. However, these possibilities are limited since such approaches are, on the one hand, associated with a relatively high effort and/or cost and, on the other hand, mutually opposed interests of the individual system suppliers have to be taken into account in the manufacture of motor vehicles. For example, the manufacturer supplying the steering system, including the hydraulic power steering system, thus usually has little influence on measures in the area of the motor.
An attempt has also already been made simply to extend the connection hose between the oil pump and the steering transmission, i.e., to provide a hose length which is larger than is actually necessary for the connection of the oil pump to the steering transmission. The irritating steering wheel vibrations can actually be minimized in a satisfactory manner or eliminated using such an additional hose length.
However, such a hose extension results in other disadvantages which are not acceptable in practice.
The connection hoses can expand due to their elasticity. The oil pressure fluctuations in the connection hose caused by the motor therefore result in corresponding hose expansion phenomena. Since these hose expansion phenomena change the flow cross-section of the hose, the result is time delays in the transfer of the oil pressure from the oil pump to the steering transmission. These delays become noticeable on steering in that a turn of the steering wheel made by the driver results in a noticeable steering torque increase. Such steering torque increases are not acceptable either with respect to driving safety or to driving comfort.
Furthermore, larger hose lengths result in problems in the so-called “packaging,” i.e. in the accommodation of the steering system and in particular of the connection hose in the motor space where increasingly less space is available so that all system suppliers are encouraged to restrict the space requirements of their components to a minimum. Larger hose lengths can therefore ultimately not be implemented in practice for this purpose.